1. Field of the Invention
This invention relates to a semiconductor laser, and more particularly to a semiconductor laser having an active region which includes at least a quantum well layer and an optical waveguide layer and is of InxGa1−xAsyP1−Y(0≦x≦1, 0≦y≦1). This invention further relates to a method of manufacturing such a semiconductor laser.
2. Description of the Related Art
A semiconductor laser has come to be used in wider and wider fields. Especially a semiconductor laser which has an GaAs substrate and oscillates in 0.7 to 1.1 μm band has come to be used, as its output power increases, not only for an optical disc and a laser printer but also for a light source for pumping a solid state laser, a fiber amplifier and a fiber laser, a primary light source for generating a second harmonic, a light source for thermally recording an image on a heat-sensitive material, for instance, in printing, a light source for medical use, a light source for laser machining and laser soldering, and the like.
In these applications, it is extremely important that the semiconductor laser can output high power. In a single mode laser which is narrow in width (not larger than about 5 μm), those which are 500 mW or more in the maximum light output and 150 mW or more in the practical light output have been reported as, for instance, a light source for pumping a fiber amplifier oscillating, for instance, at 0.98 μm or 1.02 μm. Further it has been reported that, in multiple-mode lasers which are about 50 μm or more in stripe width, the catastrophic optical damage (COD) when the oscillation wavelength is, for instance, 0.87 μm is 11.3W in the case of an element which is 100 μm in stripe width and is 16.5W in the case of an element which is 200 μm in stripe width. See “Electronics Letters”, vol. 34, No. 2, P. 184 (1998), (S. O'Brien, H. Zhao and R. J. Lang).
These inventors have proposed a semiconductor laser in which catastrophic failure due to oxidization of Al is prevented by freeing the vicinity of the light emission region (a quantum well layer and an optical waveguide layer which is adjacent to the quantum well layer and forms a barrier) from Al and at the same time, an AlGaAs layer is employed as a cladding layer in order to prevent deterioration in temperature characteristics due to leakage of electrons from the active region. With this arrangement, the semiconductor laser can operate at high output power. See “Japanese Journal of Applied Physics”, Vol. 34, No. 9B, p. L1175 (1995), (T. Fukunaga, M. Wada, H. Asano and T. Hauakawa). This will be referred to as “reference 1”, hereinbelow.
In the semiconductor laser which these inventors have proposed, the thickness of each InGaP cladding layer is 0.1 μm and the optical confinement factor (Γ) to the active layer quantum well for a laser beam is relatively large. Accordingly, when a device which was 50 μm in stripe width was aged under 500 mw at 50° C. in an APC (automatic power control) mode, deterioration rate of the drive current was relatively large and was 5×10−5h−1 in median. Further when a device which was 200 μm in stripe width was aged under 2000 mW at 25° C. in an APC mode, deterioration rate of the drive current was also 5×10−5h−1 in median.
Such high output power semiconductor lasers having a relatively wide stripe comes to stop oscillating when the drive current increases by about 5%. Accordingly the service life of the latter semiconductor laser is estimated at about 1000 hours in median, which is practically insufficient. Further since being of a gain waveguide type, the conventional semiconductor lasers are disadvantageous in that fundamental oscillation characteristics such as the current versus light output characteristics become unstable due to fluctuation in transverse mode.
Further there has been known a semiconductor laser in which the output power is increased by employing Al-free material different from that disclosed in “reference 1” and at the same time reducing the optical confinement factor (Γ) to the active layer quantum well for a laser beam by increasing the thickness of the optical waveguide layer. See “Appl. Phys. Lett.”, Vol. 72, No. 1, P. 4, (J. K. Wade, L. J. Mawst, D. Botez. R. F. Nabiev, M. Jansen and L. A. Morris) (reference 2) and “SPIE Proceeding”, Vol. 3001, p. 2 (1997), (M. A. Emanuel, J. A. Skidnore and R. J. Beach) (reference 3).
However, as disclosed in “reference 1”, especially in the wavelength range not longer than 850 nm, temperature characteristics deteriorate due to leakage of electrons into a p-type cladding layer when the cladding layer is formed of a material free from Al. This is because electron barrier cannot be sufficient even if InGaP is used which is the largest in forbidden band width in materials which can be lattice-matched with the GaAs substrate.
Further when producing such a refractive index waveguide type element, it is difficult to stop etching at the interface of an InGaP cladding layer and an InGaAsP optical waveguide layer since these layers resemble each other in chemical properties.
Further there has been reported an element in which the cladding layer is formed of InGaAlP for the purpose of suppressing deterioration in temperature characteristics due to leakage of electrons into the p-type cladding layer, as disclosed in “reference 2”. However in “reference 2”, only the gain waveguide type is mentioned and optimization for the refractive index waveguide type is not mentioned.
Further, the p-type InGaAlP is generally disadvantageous as compared with AlGaAs in that it is high in electric resistance and thermal resistance. Reliability when such a material is used has not been discussed. When the active layer is exposed to atmosphere on a GaAs substrate during production of a refractive index waveguide type element, crystallizability on the surface thereof deteriorates and remarkable deterioration of the exposed part of the crystal interface due to non-emission recombination of carriers is generated.
Accordingly, a method in which etching is carried out up to a portion immediately above the active layer has been generally employed. FIG. 2 shows a ridge waveguide type laser having an n-side electrode 20, an n-GaAs substrate 11, an n-GaAs buffer layer 12, an n-AlGaAs cladding layer 13, an undoped SCH active layer 14, a p-AlGaAs cladding layer 16, a p-GaAs capping layer 17, a SiO2 insulating film 18, and a p-side electrode 19. In this case, etching is carried out so that the upper cladding layer 16 is left in a small thickness 15 (about 0.1 to 0.3 μm) by controlling the etching time.
However such etching time control is disadvantageous in the reproducibility deteriorates due to fluctuation in etching conditions and thickness of the cladding layer from wafer to wafer. In order to overcome this problem, there has been proposed a structure in which an etching stop layer 26 as shown in FIG. 3. The structure in FIG. 3 has an n-side electrode 31, an n-GaAs substrate 21, an n-GaAs buffer layer 22, an n-AlGaAs cladding layer 23, an undoped SCH active layer 24, a p-AlGaAs cladding layer 25, a p-InGaP etching stop layer 26, a p-AlGaAs cladding layer 25, a p-GaAs capping layer 28, a SiO2 insulating film 29, and a p-side electrode 30. See U.S. Pat. No. 4,567,060 (reference 4).
For example, in the case where an AlGaAs cladding layer 23/25 and an InGaAsP active region 24 are combined, by inserting an InGaP etching stop layer 26 (about 1 to 5 nm in thickness), which is lattice-matched with the GaAs substrate 21, into the upper cladding layer 27 as shown in FIG. 3, it becomes feasible to stop etching of the AlGaAs 27 at the InGaP etching stop layer 26 in various etching methods.
However an InGaP layer inserted into a p-type AlGaAs cladding layer sometimes deteriorates crystallization, which results in increase in electric In) resistance and/or built-in voltage. This is supposed because As on the surface of the AlGaAs is substituted by P to form AlGaAsp on the crystal interface of AlGaAs and InGaP at the beginning of crystal growth.